Neutrons can be detected by the gamma ray radiation that is emitted during a neutron capture. The absorption neutron cross-section of an isotope of a chemical element is the effective cross sectional area that an atom of that isotope presents to absorption, and is a measure of the probability of neutron capture. It is usually measured in barns (b). 1 barn is equal to 10−28 m2.
The neutron temperature, also called the neutron energy, indicates a free neutron's kinetic energy, usually given in electron volts. The term temperature is used, since hot, thermal and cold neutrons are moderated in a medium with a certain temperature. The neutron energy distribution is then adopted to the Maxwellian distribution as known for thermal motion. Qualitatively, the higher the temperature, the higher the kinetic energy is of the free neutron. Kinetic energy, speed and wavelength of the neutron are related through the De Broglie relation. Fast neutrons have a kinetic energy greater than 1 eV. Slow neutrons have a kinetic energy less than or equal 0.4 eV. Thermal neutrons have a kinetic energy of about 0.025 eV. Cold neutrons have a kinetic energy from 5×10−5 eV to 0.025 eV.
Total neutron capture cross-sectional area, or absorption cross-sectional area, is the effective cross-sectional area associated with the capture of neutron by a single atom. The total neutron capture cross-sectional area is often highly dependent on neutron energy. Referring to FIGS. 1A-1I, the total neutron capture cross-sectional area as a function of incident neutron energy is plotted for 6Li, 7Li, 10B, 11B, 113Cd, 174Hf, 177Hf, 155Gd, and 157Gd. These graphs are available at a web page entitled “Evaluated Nuclear Data File (ENDF) Retrieval & Plotting,” http://www.nndc.bnl.gov/sigma/index.jsp (Last visited Jun. 22, 2010). 6Li, 10B, 113Cd, 174Hf, 177Hf, 155Gd, and 157Gd are “thermal-neutron absorbing materials,” which are materials having a total neutron capture cross-sectional area greater than 103 barns at 0.0025 eV. 7Li and 11B have a much smaller total neutron capture cross-sectional area than 6Li and 10B, respectively, illustrating that the total neutron capture cross-sectional area can vary significantly from isotope to isotope.
Examples of electromagnetic radiation than can be detected by generation of electrons include X-rays and gamma rays. Upon impinging on a matter, an X-ray or a gamma ray ionizes the matter and generates secondary electrons. In this case, the X-ray or the gamma ray can be detected by the electrons generated by the ionization upon interaction with matter.
In general, electrons generated either by zero rest mass particles (photons in the X-ray range or in the gamma ray range) or non-zero rest mass particles (such as neutrons) can be detected by a detector configured to detect the electrons that the particle generates. A common variety is a gaseous detector, which has a number of very attractive features for neutron scattering including large active area, direct conversion process, low noise and high count rate capability. However, the spatial resolution and the parallax errors of conventional gaseous neutron detectors are fundamentally limited respectively by the particle (protons and tritons) range and the conversion volume design. When a neutron is absorbed in the conversion region of the gaseous detector, charged particles are produced. These charged particles travel through the gas producing gas ionization. The range of these particles sets the spatial resolution of the detector (typically in the order of mm range) while the average number of the primary released electrons (typically of order about 30,000) determines the energy resolution. The parallax broadening occurs in all non spherical gas conversion regions where signal electrons always drift perpendicularly to the electrodes.
An elegant way to overcome these difficulties is to replace the converter from gaseous absorber to a condensed matter. A solid state with a direct conversion capability will offer all the advantages of the gaseous while reducing the spatial resolution and the parallax. Solid state neutron converter approach has been used to produce vacuum-based neutron detectors using either solid or porous materials, and they provided a better spatial resolution than a conventional gaseous detector. But solid neutron converters suffer from relatively low quantum efficiency (typically a few percent) in the neutron energy range of interest (meV-MeV range). This is due to the fact that a neutron solid converter must be relatively thick (of order tens of microns) to efficiently absorb the neutron in this range. However, the thermalization range of the electrons produced by neutron absorption is in the micron range. Thus, most of these electrons thermalize and are trapped in the converter materials. Prior art porous converters have also been successfully integrated in the current generation of vacuum-based detectors, but they suffer from several performance limitations which are inherent to the fabrication process: limited sensitive areas, limited count rates capabilities (10,000 cps/mm2) and relatively poor pulse height resolution.